Cryostat

ABSTRACT

LIQUID HELIUM FLOWS DOWNWARDLY THROUGH AN ELEMENT HAVING A PORE SIZE OF LESS THAN 10**4 CM. A CONTROLLED VACUUM IS PLACED AT THE LOWER END OF THE ELEMENT. THE LIQUID HELIUM IS COMPLETELY EVAPORATED AT THIS LOWER END. THE GASES EMITTED AT THE LOWER END HAVE A TEMPERATURE DETERMINED BY THE CONTROLLED VACUUM. IN THIS WAY, A CRYOGENIC COOLING OF A DETERMINED TEMPERATURE IS MADE AVAILABLE.

Dec. 14, 197] ELSNER El AL CRYOSTAT 2 Sheets-Sheet 2 Filed March 26 1970VIIUIIIIIIIIIIII,

rlllllllllllllllllllll FIG. 3

m am M/ N n p M E B p R n O T W m V '8. A C mm m U AG Y B Dec. 14, 1971ELSNER ETAL CRYOSTAT 2 Sheets L;he0t 1 Filed March 26, 1970 AUTOMATICCONTROL EQUIPMENT v IL FIG.)

IN VEN TOR-S.

ammw

np D. EH m CV m W mm G ATTORNEYS.

United States Patent O U.S. Cl. 62-62 15 Claims ABSTRACT OF THEDISCLOSURE Liquid helium flows downwardly through an element having apore size of less than cm. A controlled vacuum is placed at the lowerend of the element. The liquid helium is completely evaporated at thislower end. The gases emitted at the lower end have a temperaturedetermined by the controlled vacuum. In this way, a

cryogenic cooling of a determined temperature is made available.

BACKGROUND OF THE INVENTION The present invention relates to anapparatus for the low temperature cooling of various objects and to amethod of operating such apparatus. Preferably,the temperatures arebelow 2.17" K. and a replenishable helium bath is used.

Temperatures below 2.17 K. (A-point of helium) have been achieved in thepast by controlled lowering of the pressure above a bath of liquidhelium. The object to be cooled is generally submerged in the bath. Oneapparatus using such principles includes the feature of automaticreplenishment of a helium-II bath standing under reduced pressure. Twohelium baths are situated separated from one another in such a cryostat.One of these baths, the work bath, cools the object to be cooled, whilethe other bath serves for replenishment of the working bath. Treatmentof this apparatus is given in the article of A. Elsner, G. Hildebrandt,and G. Klipping, in Dechema-Monographie, volume 58 (1968), pages 9-16,and in the article of A. Elsner and G. Klipping in Advances in CryogenicEngineering, 416, volume 14 (1968). Such an apparatus makes it possibleto hold objects over extended periods of time at temperatures inthe'range below 2.17 K., but involves considerable expense, since twoliquid baths are required, as well as two pumps and two controlcircuits. Accordingly, operation is complicated and there are manypossibilities for disturbance of operation.

SUMMARY OF THE INVENTION An object of the present invention therefore isto provide a simpler apparatus for the continuous cooling of objects attemperatures down to below 2.l7 K. using a helium bath. This, as well asother objects which will become apparent in the discussion that followsare achieved according to the present invention by providing thecontainer for the helium bath with an evaporating element situated atthe lower end of the container. The evaporating element is made of aporous material having a pore size smaller than 10* cm. Suitable suchmaterial is discussed in US. Pat. No. 3,442,091, issued on May 6, 1969,to Gustav Klipping, Albrecht Elsner, and Gerd Hildebrandt for Deliveryof Coolant to Cryostats. The length of the evaporating element is chosensuch that a complete evaporation of liquid helium moving downwardlythrough the evaporating element to its external surface located in avacuum chamber is achieved. The apparatus of the present invention hasthe 3,625,?05 Patented Dec. 14, 1971 ice advantage that only one heliumbath is needed, in order to obtain extended operation at desired lowtemperatures, and that only one pump is needed. Construction andoperation of the apparatus are therefore simplified.

According to a preferred embodiment of the present invention, the lengthof the evaporating element is greater than 3 cm. This length has beenfound to give a complete evaporation of the helium flowing downwardlythrough the evaporating element-even for higher pressure above thehelium bath, for example one atmosphere.

According to another preferred embodiment of the present invention, thehelium bath is in heat exchanging relationship with the cold gas flowingfrom the evaporating element. In this way a cooling of the helium bathusing the low temperature of the cold gas is achieved. This contributesto economical operation of the apparatus.

In order to achieve a uniform cooling of the object to be cooled, it isadvantageous to provide equipment for holding the object in the flow ofgas evaporating from the external surface of the evaporating element andin direct heat conducting contact with the outer surface of theevaporating element.

In certain cases, it can be useful if the equipment for holding theobject to be cooled is a piece of high heat conductivity material, forexample copper, set into the wall of the vacuum vessel in the region ofthe external surface of the evaporating element of the invention.Besides providing means onto which an object to be cooled can bemounted, this piece of high conductivity material is generally useful asa cooling surface for various other purposes in cryogenics. In thisembodiment of the invention, the object to be cooled is separated fromthe evaporating element. Preferably, the object to be cooled is placedoutside of the coolant circulatory system, so that it can be easilygotten at, without necessitating an opening of the portion of thecryostat containing helium. It is furthermore advantageous to make thevacuum vessel, which carries the piece of high heat conductivity, out ofa material of poor heat conductivity, preferably steel.

An apparatus according to the present invention has, when compared withknown apparatus for the cooling of objects to temperatures below 2.17K., numerous advantages. It has, for example, a simpler construction,

since only one helium bath is used and one pump suf fices to obtain thedesired temperature in the object to be cooled. The temperature of thehelium bath (or in other words the pressure above the bath) does notneed to be held constant. It can vary between the boiling temperature ofhelium at one atmosphere pressure (4.2" K.) and lower values withoutinfluencing the temperature of the object to be cooled. Likewise, levelvariations of the helium bath have no influence on the temperature ofthe object. The pressure, and thus the temperature, control is done withsimpler means than those used in previously known apparatus. On thewhole, operation of the apparatus of the present invention is simplerand more certain than was the case with apparatus previously used.Furthermore, the apparatus of the present invention is simpler andcheaper to build, because of its inherently less expensive design.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a cross section of acryostat according to the present invention, the plane of the crosssection containing the cylindrical axis of the cryostat.

FIG. 2 is a view as in FIG. 1 showing an alternative embodiment of onlythe lower part of the cryostat of the present invention.

FIG. 3 is a view as in FIG. 1 showing only the lower part of analternative embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIG. 1, thefine-pored evaporating element 1 is set into container 2 for the heliumbath 3 in such a manner that the upper end of the evaporating element 1forms the floor of the container 2. The container 2 is built in the formof a heat exchanger 4 in its portion immediately above the evaporatingelement 1. This portion is intended to receive the helium bath 3.

The heat exchanger 4 has, in the particular embodiment shown in FIG. 1,the form of a ribbed tube whose ribs 5 are provided with holes spacedaround the tube and off-set with respect to one another so that it isnot possible to sight through them. According to other embodiments ofthe present invention, the heat exchanger 4 can, for example, be aporous body made using the techniques of powder metallurgy. Heatexchanger 4 is always arranged so that cold gas streaming from theexternal surface 6 of the evaporating element 1 flows onto its surfaces.In this way helium bath 3 is cooled by such cold gases.

The container 2 with the evaporating element 1 is suspended from the lid7 of the cryostat and is surrounded by a vacuum vessel 8 which is alsocarried by lid 7. A radiation protection shield 9 is attached to thevacuum vessel 8. This shield is cooled to about 100 K. by heat exchangewith the cold gas streaming from the external surface 6 through thevacuum vessel 8. It is naturally possible to use a plurality of suchradiation protection shields one around the other. Other conventionalheat insulation can likewise be used. The apparatus of the presentinvention is encased in the external cryostat housing 10, which isclosed by lid 7 and which can be evacuated through closure valve 11 toprovide an insulating vacuum.

An exhaust gas line 13 opens into the vacuum chamber 12 of vacuum vessel8. Pressure controller 14 is provided for adjusting and controlling thepressure in the vacuum chamber 12. A vacuum pump is connected to exhaustgas line 13 to bring the pressure within the vacuum chamber 12 to thatcorresponding to the temperature desired for cooling at test object. Theconstancy of the pressure in the vacuum chamber 12 is maintained usingautomatic control equipment to operate pressure controller 14. Theaction of such automatic control equipment on the pressure controller 14is indicated by dashed control line 15. The automatic control equipmentsenses the temperature of the test object, for example. The sensing ofother parameters is possible for example, pressure itself can be sensed.Various types of pressure controllers 14 With sufficiently highsensitivity can be used. The automatic control equipment is illustratedin FIG. 1 by a labelled rectangular box, since its detailed illustrationis not essential for a proper understanding of the invention.

Additionally passing through lid 7 is a vacuum-jacket syphon 16 carryingan expansion valve 17 on its lower end. Suitable examples of suchsyphons are shown in US. Pat. No. 3,442,091, issued to Gustav Klipping,Albrecht Elsner, and Gerd Hildebrandt on May 6, 1969, for Delivery ofCoolant to Cryostats. The uppermost arrow in FIG. 1 indicates the flowof liquid helium down through a pipe in A the center of thevacuum-jacket syphon 16 while the arrow directed upwardly just below theuppermost arrow indicates the flow of exhaust .gas from helium bath 3flowing through the exhaust gas tube C in the outer jacket D of thevacuum-jacket syphon 16. As the exhaust gas tube in the outer jacket isconnected to a radiation shield B surrrounding the liquid carrying pipeA, the radiation shield B thus being cooled, a radiation protection forliquid helium flowing in the central pipe is achieved. Level sensor 18emits a signal to open expansion valve 17 when the level in helium bath3 has fallen 4 a predetermined amount and liquid helium is brought intothe helium bath 3 from a storage vessel, for example, in the mannershown in U.S. Pat. No. 3,442,091.

Bracket 1% serves for the mounting of test objects to be cooled. Theconnection of bracket 19a to the external surface 6 of evaporatingelement 1 is such that heat is easily conducted between the evaporatingelement 1 and the bracket 19a. Depending upon the particular type ofmaterial used to construct evaporating element 1, the mounting ofbracket 19a can be effected by gluing, soldering, brazing, etc, Toobtain a good heat conducting connection of the test object to thebracket 19a the test object 29 can be screwed, soldered, brazed, orglued to the bracket 19a.

Various materials can be used to make the fine-pored evaporating element1, provided they have pore sizes less than 10- cm. Examples are aluminumsilicate, alumina, coal, fritted glass, and bodies of sintered nickel,silver, and copper powders. The length of the evaporating element 1 fromthe floor of the helium bath 3 to the external surface 6 depends firstlyon the maximum pressure to be expected in container 2 above the heliumbath 3 and on the various temperatures desired for the test object.Especially at working temperatures near 217' K., the exceeding ofcertain pressure values in the container 2 above the bath 3 can lead toan undesired flow of fluid from the external surface 6 of theevaporating element 1, should the length of the evaporating element 1 betoo small. Lengths of more than 3 cm. have been found to eliminate thispossibility with certainty.

FIG. 2 shows an alternate embodiment of the lower part of FIG. 1. Thisalternate embodiment provides a different means for holding a testobject. Thus, in place of bracket 19a there is provided plate 19b, whichis so placed in the floor of vacuum vessel 8, that it is directly in thepath of cold gas evaporating from the external surface 6 of evaporatingelement 1. A test object 30 mounted on the bottom side of this plate 1%in the manner given above for bracket 19a is thus cooled and held at thedesired working temperature. Plate 191) is formed from a material havinga high heat conductivity, for example copper, while the vacuum vessel 8itself is made of a poor heat conducting material, for example steel.

In this embodiment of FIG. 2, the radiation protection shield 9 ispreferably designed so that it can be removed from the vacuum vessel 8.Then, the test object to be mounted on the plate 19b can be most easilyreached by removal of the cryostat housing 10 and the radiationprotection shield 9. The parts of the apparatus of the present inventionbelonging to the helium circulation system do not in this case need tobe opened.

FIG. 3 shows a third embodiment of the lower part of the apparatus ofthe present invention. Plate 20, which forms the floor of vacuum vessel8 and which is made of copper, is likewise placed in the path of coldgases evaporating from the external surface 6 of the evaporatingelement 1. Plate 20 is used here to form a cryopump and provides a coldsurface for the condensation of gases. Accordingly, the surroundingradiation protection shield 9 is provided in the region of its floorWith a Well-known chevron system 21 which is likewise cooled by heatconductivity. A larger receptacle serves as the cryostat housing 10 andthis is evacuated with the help of the cryopump action of plate 20.

In operation, the helium bath 3 is filled and replenished by way ofsyphon 16 from a storage vessel. The expansion valve 17 may be operatedby hand or preferably automatically with the help of level sensor 18.The vacuum in vacuum vessel 8 is brought to the particular pressurecorresponding to the desired working temperature for the test object andthen held constant. For example, for a temperature of T=1.7 K., apressure of 8.6 mm. of mercury is used. The liquid helium from the bath3, above which the pressure is higher (for example, pressure=atmosphericpressure) than the pressure present in vacuum chamber 12, flows throughthe fine-pored evaporating element and completely evaporates on theexternal surface 6 of the evaporating element 1. As a result of thisevaporation, a cooling occurs at the external surface 6 of theevaporating element 1, until the temperature corresponding to thepressure in vacuum vessel 8 is reached. Consequently, a temperaturegradient arises in the evaporating element 1 and in the liquid heliumheld in its pores. The cold helium gas arising at the external surface 6of the evaporating element 1 streams through the heat exchanger 4, thuscooling the helium bath 3, and finally is pumped out through exhaustline 13. The offset of the holes of heat exchanger 4 relative to oneanother provides improved contacting of the ribs 5 by the cold gas. Theparticular working temperature can be held constant as long as desired.

Since the helium bath can be held to a very small volume (for example,to 50 cmfi), the apparatus is no usual bath cryostat. It exhibits theadvantages of a helium evaporation cryostat. Upon ending or interruptingoperation of the apparatus of the present invention, only minimalcoolant losses occur and it is possible to interrupt operation withoutthe time loss determined in bath cryostats by the evaporation of thecoolant. Minimum cooling times are achieved and a minimum consumption ofcoolant for cooling results, since there are no dead volumes to becooled. Coolant consumption during steady-state operation iscorrespondingly small. A continual operation of any desired length oftime is possible because of the simple yet exact manner in which theapparatus of the present invention operates.

It will be understood that the above description of the presentinvention is susceptible to various modifications, changes andadaptations, and same are intended to be comprehended within the meaningand range of equivalents of the appended claims.

We claim:

1. A method suitable for the cooling of objects at temperatures below2.l7 K. using a replenishable helium bath, comprising the steps offilling liquid helium into a container to form a bath, the floor ofwhich container is an evaporating element having pores of a size lessthan 10- cm., creating a vacuum on the side of said evaporating elementopposite to that contacting said helium bath, and completely evaporatinghelium moving downwardly through said element from said container as itpasses from said element into said vacuum.

2. A method as claimed in claim 1, further comprising the step oftransferring heat from said bath to gaseous helium evaporated from saidevaporating element.

3. A method as claimed in claim 1, further comprising the step ofplacing an object to be cooled in heat conductive contact with the sideof said evaporating element opposite to that contacting said heliumbath.

4. A method as claimed in claim 1, further comprising the step ofbounding the vacuum With a vacuum vessel and with a plate situated inthe flow of helium evaporated from said evaporating element and formingone portion of the walls of said vacuum vessel.

5. A method as claimed in claim 4, the heat conductivity of said platebeing higher than that of the remainder of said vessel.

6. A method as claimed in claim 5, further comprising the step ofmounting an object to be cooled in heat conductive contact With the sideof said plate opposite to said vacuum.

7. A method as claimed in claim 5, further comprising the step ofcondensing gas on the side of said plate opposite to said vacuum, in thepresence of the interior of an evacuable housing.

8. Apparatus for the cooling of objects, suitable for temperatures below2.l7 K. using a replenishable helium bath, comprising a vacuum chamber,a helium bath container extending downwardly into said vacuum chamber,an evaporating element situated within said vacuum chamber and formingthe lower end of said helium bath container, said evaporating elementhaving a pore size smaller than 10 cm., the length of said evaporatingelement from the lower surface of a helium bath in said container to theface of said element exposed to the interior of the vacuum chamber beingsuch that helium moving downwardly through the evaporating element tosuch face completely evaporates at such face.

9. An apparatus as claimed in claim 8, said length of the evaporatingelement being greater than 3 cm.

10. An apparatus as claimed in claim 8, further comprising means fortransferring heat from a helium bath in said container to heliumevaporated at such face.

11. An apparatus as claimed in claim 8, further comprising means forholding an object to be cooled in the gas evaporating from such face andin direct heat conductive contact with such face.

12. An apparatus as claimed in claim 8, further comprising means in thewall of said vacuum chamber situated in the fiow of helium gasevaporating from such face for providing a cold piece.

13. An apparatus as claimed in claim 12, said piece being of material ofhigher heat conductivity than the remainder of said vacuum chamber.

14. An apparatus as claimed in claim 13, further comprising an object inheat conductive contact to said piece on the side of said piece oppositeto the interior of said vacuum chamber.

15. An apparatus as claimed in claim 13, further comprising an evacuablehousing and means for mounting said evacuable housing relative to saidpiece such that the interior of said evacuable housing is incommunication with the side of said piece opposite to the interior ofsaid vacuum chamber, whereby a cryopump is created.

References Cited UNITED STATES PATENTS 3,424,230 1/1968 Wright 62-5143,391,546 7/1968 Campbell 62514 3,410,110 12/1968 Hoyes 62514 3,447,3336/ 1968 Goodstein 625 14 MEYER PERLIN, Primary Examiner US. Cl. X.R.62-514

